In the manufacture of plastic ball grid array (PBGA) packages, a chip is electrically and mechanically attached to a laminated, organic substrate. The thermal expansion of the material of the chip is not usually compatible with that of the organic laminate material. The mismatch in coefficients of thermal expansion (CTE) and contraction causes the module to warp, and leads to fatigue failure of the BGA.
In order to maintain the flatness of a PBGA package, especially those with a flip-chip, the chip may be overlaid with a heat spreading cap, which has been designed to balance the CTE and stiffness of the substrate that is disposed on the opposite side of the chip. However, the heat spreading cap has been only partially successful in maintaining flatness because it correctly balances the substrate only directly above the chip.
Another source of package warping has been determined to be the epoxy adhesive that cements the chip and cap to the substrate. The epoxy adhesive, which extends beyond the periphery of the cap, may not match the CTE of the substrate. Also, it has been observed that the epoxy tends to absorb and desorb moisture, causing additional expansion and contraction and consequent warpage of the module.
The present invention represents means by which the combined thermal mismatches between the chip, cap, adhesive, and laminated substrate can be overcome to maintain flatness over the operating temperature range, thereby increasing the fatigue life of the BGA.
The current inventors seek to vary the thickness, shape, or materials of the cap where it extends beyond the periphery of the chip. This variation in thickness, material, or shape resolves the mismatch problem by changing either the stiffness, the thermal expansion, or both, in the affected regions. The objective can also be achieved by the introduction of holes, grooves, and multiple materials in the cap(s).
Holes, fins, or separating borders may be fabricated by drilling, punching, shaping, etching, etc. to vary the stiffness.
The methods of the invention have been found to significantly reduce module warpage and to increase the fatigue life of the BGA while maintaining good heat dissipation, thus providing a structurally reliable module.